Chiral compounds nomenclature systems

To facilitate the identification of the absolute configuration of the phosphorus center, the nomenclature system for a coordination compound [MX(AB)2] (AB = hetero-bidentate ligand) that can have TBP or square-pyramidal (SP) geometry was used. In the TBP geometry, when a monodentate ligand X occupies one equatorial position, the chiral-at-metal configuration can be defined as /I or d (Figure 3). 

This chapter will introduce the concepts of asymmetry and chirality as they apply to stereoisomers. Enantiomers are nonsuperimposable mirror images that are different compounds, identifiable only by differences in the physical property known as specific rotation. Enantiomers arise when a molecule has one or more atoms (including carbon) with different substituents (from different substituents for carbon). Such atoms are known as stereogenic (chiral) atoms (most of the examples in this book will deal with a stereogenic carbon atom). With more than one stereogenic center, another type of stereoisomer results known as a diastere-omer. All of these are types are stereoisomers, and a nomenclature system is in place to correlate the structure with what is known as absolute configuration. 

Systems of Nomenclature for Chiral Compounds The two enantiomers of a chiral drug or natural product are best identified on the basis of their absolute configuration [(/ /5)-system] or their optical rotation... 

Many chiral molecules lack a conventional center that can be described by the R/S or E/Z nomenclature system. Typically these molecules can be viewed as helical, and they may have propeller, or screw-shaped, structures. For example, conformations of simple chain compounds can also be treated as if they had helical chirality. 

The nomenclature for biaryl, allene, or cyclohexane-type compounds follows a similar rule. Viewed along the axis, the nearer pair of ligands receives the first two positions in the order of preference, and the farther ligands take the third and fourth position. The nomination follows a set of rules similar to those applied in the central chiral system. In this nomination, the end from which the molecule is viewed makes no difference. From whichever end it is viewed, the positions remain the same. Thus, compound 7a has an ( -configuration irrespective of which end it is viewed from. 

Given the importance of stereochemistry in reactions between biomolecules (see below), biochemists must name and represent the structure of each biomolecule so that its stereochemistry is unambiguous. For compounds with more than one chiral center, the most useful system of nomenclature is the RS system. In this system, each group attached to a chiral carbon is assigned a priority. The priorities of some common substituents are... 

The chiral identity of a molecule is included in the nomenclature of inorganic compounds, and today s comprehensive system is based upon suggestions made in 1990 in IUPAC s Recommendations on Nomenclature of Inorganic Chemistry [84], and ACS s Inorganic Chemical Nomenclature [90]. The basis for the usage of stereochemical descriptors was laid by Brown [91,92], from which three types of chiral descriptor conventions were developed (i) Steering-wheel-convention [93], (ii) Skew-lines convention [94] and (iii) Oriented-skew-lines convention [95]. 

In addition to the R and S designations, compounds with two chiral centers may also be identified by stereochemical nomenclature that describes the entire system. For example, the erythro and threo nomenclature derived from carbohydrate chemistry may be employed to describe the relative positions of similar groups on each chiral carbon. Thus, the ephedrines are designated as erythro forms since the similar groups (OH and NHCH3) are on the same side of the vertical axis of the Fischer projection, and the pseudo-ephedrines are designated as threo forms since like groups are on opposite sites of the vertical axis of the projection (Fig. 10). 

In the stereochemical nomenclature of coordination compounds, the procedure for assigning priority numbers to the ligating atoms of a mononuclear coordination system is based upon the standard sequence rules developed for chiral carbon compounds (the Cahn, Ingold, Prelog or CIP rules6, see Section IR-9.3.3.2). 

Figure 2.36 The nomenclature for the regiochemistry of additions must rely on a more complex system if (5,6)-adducts are to be considered, too. Moreover, the compound s chirality must be regarded for the derivatives concerned.

According to the traditional system of carbohydrate nomenclature, the configuration of any monosaccharide having up to six carbon atoms is completely defined by a prefix D or L followed by a syllable glue- , mann- etc. This latter specifies both the number of carbon atoms in the sugar and the relative stereochemistry of all chiral centres, while the D or L prefix designates the enantiomeric series to which the compound belongs. 

Use the R, S system of nomenclature to name chiral organic compounds. 

---